Becky Watts went missing in February this year.
Photograph: Avon and Somerset police/PA

Nathan Matthews, the man accused of murdering Becky Watts, hated the Bristol teenager because of the way she treated his mother and found her “spoiled and selfish”, a jury has been told.

Matthews was also controlling and aggressive towards his partner, Shauna Hoare, who is alleged to have been involved in a plot to kidnap and kill his 16-year-old stepsister Becky, the court heard.

Giving evidence for the prosecution, Hoare’s best friend, 22, who asked not to be named, revealed she had taken part in “threesomes” with Hoare and Matthews that had been filmed on a mobile phone by him.

She took part in the consensual sex with the couple after the subject had been raised jokingly by her own boyfriend because she spent so much time with the couple, the court heard.

Choking back sobs, she said: “He joked: ‘You are always there: are you having a threesome?’ We would laugh because it was so ridiculous.” Around a year before Becky was killed, the joke became a reality, the jury was told.

The woman said Matthews would get quite agitated when he talked about Becky. The witness said Matthews felt the 16-year-old treated his mother – Becky’s stepmother, Anjie Galsworthy – “like crap”.

She also told the jury Matthews controlled what Hoare ate and made her pay him a fee when she wanted to smoke a cigarette. She said Hoare was “too scared” to leave Matthews.

The jury at Bristol crown court has been told that Matthews, 28, and Hoare, 21, murdered Becky at her home in Bristol, took her body to their house two miles away, dismembered her body and hid the remains in a neighbour’s garden shed.

According to the prosecution, the murder was sexually motivated and the pair had a fascination with the idea of kidnap, “threesomes”, and an unnatural interest in young girls.

Matthews, a delivery driver and former Territorial Army soldier, admits the manslaughter of Becky, dismembering her body and possessing two stun guns. He denies conspiracy to kidnap and murder. Hoare denies any involvement in the plot to kidnap, the murder or the aftermath.

Under cross-examination from Matthews’s barrister, Adam Vaitilingam QC, Hoare’s best friend described how she went on holiday to Butlin’s with the couple and Becky in August 2014, six months before the 16-year-old was killed.

“Everyone was fine, everything was normal,” she said. But the woman agreed that Matthews “hated” Becky. She added: “He hated the way she treated his mum. He said she [Becky] was a brat … Nathan said she treated his mum like crap, that she was spoiled and selfish.”

Questioned by Hoare’s lawyer, Andrew Langdon QC, the woman said Matthews forbade his partner from eating fattening food or drink. She said Matthews made Hoare pay him if she wanted a cigarette and charged her when he drove her.

The woman said she and Hoare devised a plan for her to leave Matthews but her friend would not go through with it. “She was scared that he would kick off,” she said. “She was interested [in leaving him] but I think she was too scared to do it.”

Hoare’s friend said Matthews was verbally aggressive towards his partner. She had heard him say: “Shut the fuck up, you twat.” She also said Hoare had once shown her a bruise on the bottom of her back.

A saw identical to the one allegedly used to dismember Becky was demonstrated in court, after the judge asked how loud it was.

Forensic engineer Prof Sarah Hainsworth, who examined Becky’s bones and tools allegedly used to dismember her, said the lack of “false-start cuts” indicated the person who cut up the teenager’s body was either “careful” – or suggested that two people were involved.